The present invention relates to an apparatus for mounting an optical retarder or wave plate.
A retarder or wave plate is an optical element that changes the polarization of an incident light wave. In principle, the retarder causes the phase of one of the constituent coherent polarization states to lag behind the other by a predetermined amount. For example, a quarter-wave plate introduces a relative phase shift of 90 degrees (π/2) between the constituent orthogonal o- and e- components of a light wave. Quarter-wave plates are commonly used to convert elliptically polarized light to linear polarized light or transform linear polarized light to circular polarized light. Another common wave plate is the half-wave plate. The half-wave plate introduces a relative phase shift of 180° (π) to incident polarized light. Half-wave plates are commonly used to rotate the polarization vector of linear polarized light or to convert the sense of circular polarized light.
An exemplary use of a quarter-wave plate is in the construction of a liquid crystal light valve projector such as that disclosed by Schmidt et al. in U.S. Pat. No. 5,576,854. Schmidt et al. disclose that the perceived quality of a projected image can be improved by making the “off” or black state as black as possible. This increases the contrast between light and dark areas of the projected image. In a liquid crystal light valve projector, light from a source is projected onto a polarizer which reflects S-polarized light to a liquid crystal panel. If a part of the liquid crystal light panel is in the “on” state, the liquid crystals become birefringent and convert the S-polarized light to P-polarized light which is reflected back to the polarizer. This light passes through the polarizer and a lens and then on to a projection screen to produce a “lighted” condition on the screen. If the light valve is in its “off” state, a mirror behind the liquid crystal layer reflects the S-polarized light back onto the polarizing surface. S-polarized light striking the polarizer is reflected from the polarizer in the direction of the light source and away from the viewing screen to produce a “black” screen condition.
However, the polarization process is not perfectly efficient and the generally S-polarized light reflected from the polarizer includes a small portion of light that is not S-polarized with respect to the polarizer. In other words, the generally S-polarized light reflected onto the polarizing surface has a component in the P-polarized direction. When this P-polarized component is reflected by the liquid crystal panel to the polarizer, it leaks through the polarizer and onto the projection screen. As a result, the screen is partially illuminated when the light panel is “off” and the screen is supposed to be dark. A quarter-wave plate with its fast axis either perpendicular or parallel to the axis of polarization of the polarizing surface is located between the polarizer and the liquid crystal panel. The phase of the P-polarized component is shifted 180° as the light passes through the quarter-wave plate and then is reflected back through the wave plate by the liquid crystal panel. As a result the light reflected from the “off” liquid crystal panel will be effectively 100% S-polarized when it impinges on the polarizer. Since the light no longer includes a component that can leak through the polarizer the screen will be darker improving the contrast between light and darker areas of the image.
Wave plates are typically produced from uniaxial materials having a single optic axis and two indices of refraction. Light entering the material is divided into two waves which emerge from the material along two axes. Light polarized along the direction of the axis exhibiting the smaller index of refraction travels faster and, therefore, this axis is termed the fast axis. The second axis or slow axis exhibits the larger index of refraction and light polarized along the direction of this axis travels slower. Since a quarter-wave plate produces a phase shift of 90° between perpendicular fast and slow axes, the wave plate has heretofore been considered to be substantially optically symmetrical about either the fast or slow axis. In other words, 180 degree rotation of the quarter-wave plate about the normal to the intersection of the fast and slow axes has been expected to produce the same optical effect.